U.S. patent application number 13/091174 was filed with the patent office on 2012-01-19 for multiple wavelength receiver module.
This patent application is currently assigned to MITSUBISHI ELECTRIC CORPORATION. Invention is credited to Tatsuo Hatta, Keita Mochizuki, Takuro Shinada.
Application Number | 20120012738 13/091174 |
Document ID | / |
Family ID | 45466180 |
Filed Date | 2012-01-19 |
United States Patent
Application |
20120012738 |
Kind Code |
A1 |
Shinada; Takuro ; et
al. |
January 19, 2012 |
MULTIPLE WAVELENGTH RECEIVER MODULE
Abstract
A multiple wavelength light detector module includes an optical
fiber emitting an optical signal including light of multiple
wavelengths, a prism on which the optical signal is incident, a
total reflection mirror bonded to a first surface of the prism, a
bandpass filter bonded to a second surface of the prism, opposite
the first surface, and a photodetector for detecting optical beams
exiting the bandpass filter. The first surface extends at an angle
with respect to the second surface. When light is incident on the
bandpass filter, the bandpass filter transmits only light of a
particular wavelength determined by the angle of incidence of the
light, and reflects light of remaining wavelengths in the
light.
Inventors: |
Shinada; Takuro; (Tokyo,
JP) ; Hatta; Tatsuo; (Tokyo, JP) ; Mochizuki;
Keita; (Tokyo, JP) |
Assignee: |
MITSUBISHI ELECTRIC
CORPORATION
Tokyo
JP
|
Family ID: |
45466180 |
Appl. No.: |
13/091174 |
Filed: |
April 21, 2011 |
Current U.S.
Class: |
250/226 |
Current CPC
Class: |
G02B 6/29367 20130101;
G02B 6/26 20130101; H04B 10/675 20130101; G02B 6/4215 20130101 |
Class at
Publication: |
250/226 |
International
Class: |
G01J 3/18 20060101
G01J003/18; G01J 3/50 20060101 G01J003/50 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 14, 2010 |
JP |
2010-160015 |
Claims
1. A multiple wavelength light detector module comprising: an
optical fiber for emitting an optical signal including light of a
plurality of wavelengths; a prism on which the optical signal is
incident; a total reflection mirror bonded to a first surface of
said prism; a bandpass filter bonded to a second surface of said
prism, said second surface being opposite said first surface; and a
photodetector for detecting optical beams exiting said bandpass
filter, wherein said first surface extends at an angle with respect
to said second surface, and when light is incident on said bandpass
filter, said bandpass filter transmits only light of a particular
wavelength determined by angle of incidence of the light that is
incident on said bandpass filter, and reflects light of other
wavelengths in the light that is incident on the bandpass
filter.
2. The multiple wavelength light detector module according to claim
1, further comprising a condenser lens disposed where the optical
beams exiting said bandpass filter cross one another.
3. The multiple wavelength light detector module according to claim
1, further comprising a plurality of said photodetectors on a
single substrate.
4. The multiple wavelength light detector module according to claim
3, wherein: light detecting portions of said plurality of
photodetectors are linearly aligned on said substrate in a
direction to detect the optical beams exiting said bandpass filter
at different angles, the optical beams having been demultiplexed
from the optical signal by said bandpass filter; and at least one
of said light detecting portions is elliptical in shape, and has a
major axis extending in the direction in which said light detecting
portions are aligned on said substrate.
5. The multiple wavelength light detector module according to claim
3, wherein: light detecting portions of said plurality of
photodetectors are linearly arranged on said substrate in a
direction to detect the optical beams exiting said bandpass filter
at different angles, the optical beams having been demultiplexed
from the optical signal by said bandpass filter; and an
intermediate one of said light detecting portions linearly arranged
on said substrate is smaller in area than all other of said light
detecting portions.
6. The multiple wavelength light detector module according to claim
3, wherein: light detecting portions of said plurality of
photodetectors are linearly arranged on said substrate in a
direction to detect the optical beams exiting said bandpass filter
at different angles, the optical beams having been demultiplexed
from the optical signal by said bandpass filter; and the two
outermost light detecting portions of said light detecting portions
linearly arranged on said substrate are smaller in area than all
other of said light detecting portions.
7. The multiple wavelength light detector module according to claim
3, wherein the light detecting portions of said plurality of
photodetectors have differing areas and said light detecting
portions having larger areas detect optical beams which travel a
longer path to said light detecting portions than the optical beams
detected by said light detecting portions having smaller areas.
8. The multiple wavelength light detector module according to claim
3, wherein one of said light detecting portions of said plurality
of photodetectors is smaller in area than all other of said light
detecting portions.
9. The multiple wavelength light detector module according to claim
3, comprising a condenser lens array disposed between said bandpass
filter and said plurality of photodetectors, said condenser lens
array including a plurality of condenser lenses on a substrate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a multiple wavelength
receiver module adapted to receive an optical signal including
light of a plurality of wavelengths.
[0003] 2. Background Art
[0004] Japanese Laid-Open patent Publication No. 2004-85860
discloses a multiple wavelength receiver module adapted to receive
an optical signal including light of a plurality of wavelengths
(which signal is referred to as a multiplexed optical signal). In
this module, the received optical signal is separated, or
demultiplexed, into optical beams (or optical signals) each having
a different one of the wavelengths, which beams are then received
by photodetectors. The module includes a plurality of filters for
such demultiplexing of the multiplexed optical signal. Each filter
is adapted to allow only light of a particular different wavelength
to pass therethrough. That is, there are as many filters as there
are wavelengths in the multiplexed optical signal.
[0005] The multiple wavelength receiver module further includes, on
the exit side of the filters, condenser lenses for converting the
separated optical beams into focused beams. Each condenser lens is
associated with a different one of the separated optical beams.
Therefore, there must be at least as many condenser lenses as there
are separated optical beams. The multiple wavelength receiver
module further includes, on the exit side of the condenser lenses,
photodetectors for receiving or detecting the optical beams
emerging from the condenser lenses. When this multiple wavelength
receiver module, constructed as described above, is assembled, the
optical axes of the condenser lenses and the photodetectors are
adjusted (that is, aligned with each other) to reduce the optical
loss in the module. This adjustment is referred to as "optical axis
alignment."
[0006] In the multiple wavelength receiver module disclosed in the
above publication, each, filter and each condenser lens are adapted
to handle a different one of the wavelengths in the multiplexed
optical signal. This means that the more wavelengths in the
multiplexed optical signal, the more filters and condenser lenses
required, and hence the higher the cost of the multiple wavelength
receiver module.
[0007] Further, multiple wavelength receiver modules with a
plurality of condenser lens, such as the multiple wavelength
receiver module disclosed in the above publication, require
complicated assembly operation, since the optical axis of each
condenser lens must be aligned separately.
SUMMARY OF THE INVENTION
[0008] The present invention has been made to solve the above
problems. It is, therefore, an object of the present invention to
provide a low-cost multiple wavelength receiver module which is
easy to assemble.
[0009] According to one aspect of the present invention, a multiple
wavelength receiver module includes an optical fiber for emitting
an optical signal including light of a plurality of wavelengths, a
prism disposed to receive the optical signal, a total reflection
mirror bonded to a first surface of the prism, a bandpass filter
bonded to a second surface of the prism opposite the first surface,
and a photodetector for receiving optical beams exiting the
bandpass filter. The first surface extends at an angle with respect
to the second surface. When light is incident on the bandpass
filter, the bandpass filter transmits only light of a particular
wavelength determined by the angle of incidence of the incident
light, and reflects light of the remaining wavelengths in the
incident light.
[0010] Other and further objects, features and advantages of the
invention will appear more fully from the following
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 a cross-sectional view of a multiple wavelength
receiver module in accordance with a first embodiment of the
present invention;
[0012] FIG. 2 is an enlarged view of the three-layer filter;
[0013] FIG. 3 is a diagram showing the light receiving surface of
the photodetector array;
[0014] FIG. 4 is a cross-sectional view of the multiple wavelength
receiver module, showing the internal light paths;
[0015] FIG. 5 is a diagram showing the light paths within the
prism;
[0016] FIG. 6 is a diagram showing the paths traveled by these
beams from the bandpass filter to the light receiving portions of
the photodetectors;
[0017] FIG. 7 is a variation of the photodetector array of the
first embodiment;
[0018] FIG. 8 is a diagram showing a variation of the total
reflection mirror of the first embodiment;
[0019] FIG. 9 is a diagram showing the light receiving surface of
the photodetector array of the multiple wavelength receiver module
of the second embodiment;
[0020] FIG. 10 is a diagram showing a variation of the
photodetector array; and
[0021] FIG. 11 is a diagram showing a multiple wavelength receiver
module in accordance with a third embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First Embodiment
[0022] FIG. 1 is a cross-sectional view of a multiple wavelength
receiver module 10 in accordance with a first embodiment of the
present invention. The multiple wavelength receiver module 10
includes a holder 12. Components of the multiple wavelength
receiver module 10 are secured to the holder 12. Specifically, an
optical fiber 14 is fixed to the holder 12. A multiplexed optical
signal is introduced into the holder 12 through the optical fiber
14. A collimator lens 16 is secured to the holder 12 on the exit
side of the optical fiber 14. The collimator lens 16 is used to
collimate the multiplexed optical signal.
[0023] A prism 18 is secured to the holder 12 on the exit side of
the collimator lens 16. The prism 18 has a first surface 18a and an
opposing second surface 18b. A total reflection mirror 20 is bonded
to the first surface 18a. The total reflection mirror 20 reflects
light at all the wavelengths. The total reflection mirror 20 has an
opening 20a. The opening 20a is disposed to introduce the
collimated multiplexed optical signal from the collimator lens 16
into the prism 18.
[0024] A bandpass filter 22 is bonded to the second surface 18b of
the prism 18. The bandpass filter 22 transmits light of a different
wavelength depending on the angle of incidence. That is, when the
multiplexed optical signal is incident on the bandpass filter 22 at
some angle of incidence, the filter 22 transmits only light of a
particular wavelength determined by that angle of incidence; and
when the multiplexed optical signal is incident on the bandpass
filter 22 at a different angle of incidence, the filter 22
transmits only light of a different wavelength. This characteristic
of the bandpass filter 22 is referred to as the angle-of-incidence
dependence of the transmission of the bandpass filter 22. It should
be noted that the prism 18, the total reflection mirror 20, and the
bandpass filter 22 are sometimes referred to collectively as the
three-layer filter.
[0025] A condenser lens 24 is secured to the holder 12 on the exit
side of the three-layer filter. The condenser lens 24 is a
single-piece lens. The condenser lens 24 converts the collimated
optical beams exiting the bandpass filter 22 into focused
beams.
[0026] A photodetector array 26 is secured to the holder 12 on the
exit side of the condenser lens 24. The photodetector array 26
includes a substrate 26s and photodetectors 26a, 26b, 26c, and 26d
monolithically secured to the substrate 26s.
[0027] FIG. 2 is an enlarged view of the three-layer filter. The
prism 18 has a wedge configuration. Specifically, the first surface
18a extends at an angle with respect to the second surface 18b.
That is, the width W1 of the upper end of the prism 18 is greater
than the width W2 of the lower end of the prism 18.
[0028] FIG. 3 is a diagram showing the light receiving surface of
the photodetector array 26. The light receiving portions of the
photodetectors 26a, 26b, 26c, and 26d of the photodetector array 26
increase in area in the order named (that is, the light receiving
portion of the photodetector 26d is the largest in area and the
light receiving portion of the photodetector 26a is the smallest).
The light receiving portions of the photodetectors 26a, 26b, 26c,
and 26d are linearly arranged on the substrate 26s in such a
direction as to receive the separated optical beams emerging from
the bandpass filter 22 at different angles.
[0029] FIG. 4 is a cross-sectional view of the multiple wavelength
receiver module 10, showing the internal light paths. The following
description will be directed to these internal light paths. First,
a multiplexed optical signal including light of wavelengths
.lamda.1, .lamda.2, .lamda.3, and .lamda.4 is emitted from the
optical fiber 14. This multiplexed optical signal is collimated by
the collimator lens 16. The collimated optical signal is introduced
into the prism 18 through the opening 20a of the total reflection
mirror 20. The light paths within the prism 18 will be described
with reference to FIG. 5.
[0030] FIG. 5 is a diagram showing the light paths within the prism
18. When the multiplexed optical signal is incident on the bandpass
filter 22 for the first time (hereinafter referred to as the "first
incidence event"), only light of wavelength .lamda.1 in the
incident light passes through the bandpass filter 22 and light of
the remaining wavelengths is reflected by the bandpass filter 22 at
an angle of reflection .theta.1 due to the angle-of-incidence
dependence of the transmission of the bandpass filter 22.
[0031] The light reflected from the bandpass filter 22 at the angle
of reflection .theta.1 is then totally reflected by the total
reflection mirror 20 at an angle .theta.2 (hereinafter referred to
as the "first total reflection event"). This totally reflected
light is then incident on the bandpass filter 22 (hereinafter
referred to as the "second incidence event"). The angle of
incidence in the second incidence event differs from that in the
first incidence event, since the incident light in the second
incidence event is light totally reflected by the total reflection
mirror 20, which mirror extends at an angle with respect to the
bandpass filter 22. Therefore, in the second incidence event, only
light of wavelength .lamda.2 in the incident light passes through
the bandpass filter 22 and light of the remaining wavelengths in
the incident light is reflected by the bandpass filter 22 at an
angle of reflection .theta.3 due to the angle-of-incidence
dependence of the transmission of the bandpass filter 22.
[0032] The light reflected from the bandpass filter 22 at the angle
of reflection .theta.3 is then totally reflected by the total
reflection mirror 20 at an angle .theta.4 (hereinafter referred to
as the "second total reflection event"). This totally reflected
light is then incident on the bandpass filter 22 (hereinafter
referred to as the "third incidence event"). The angle of incidence
in the third incidence event differs from those in the first and
second incidence events, since the incident light in the third
incidence event is light totally reflected twice by the total
reflection mirror 20, which mirror extends at an angle with respect
to the bandpass filter 22. Therefore, in the third incidence event,
only light of wavelength .lamda.3 in the incident light passes
through the bandpass filter 22 and light of the remaining
wavelengths is reflected by the bandpass filter 22 at an angle of
reflection .theta.5 due to the angle-of-incidence dependence of the
transmission of the bandpass filter 22.
[0033] The light reflected from the bandpass filter 22 at the angle
of reflection .theta.5 is then totally reflected by the total
reflection mirror 20 at an angle .theta.6 (hereinafter referred to
as the third total reflection event"). This totally reflected light
is then incident on the bandpass filter 22 (hereinafter referred to
as the "fourth incident event"). The angle of incidence in the
fourth incidence event differs from those in the first, second, and
third incidence events, since the incident light in the fourth
incidence event is light totally reflected three times by the total
reflection mirror 20, which mirror extends at an angle with respect
to the bandpass filter 22. Therefore, in the fourth incidence
event, light of wavelength .lamda.4 in the incident light passes
through the bandpass filter 22 due to the angle-of-incidence
dependence of the transmission of the bandpass filter 22.
[0034] In this way the multiplexed optical signal including light
of four wavelengths (namely, wavelengths .lamda.1, .lamda.2,
.lamda.3, and .lamda.4) is separated, or demultiplexed, into four
optical beams having wavelengths .lamda.1, .lamda.2, .lamda.3, and
.lamda.4, respectively. In FIG. 5, the reference symbols .lamda.1,
.lamda.2, .lamda.3, and .lamda.4 denote the separated optical
beams, and .theta.1', .theta.3', .theta.5', and .theta.7' denote
the exit angles of these separated optical beams .lamda.1,
.lamda.2, .lamda.3, and .lamda.4, respectively, from the bandpass
filter 22. Thus, the incident light entering the prism 18 travels
from the wide upper side toward the narrow lower side of the prism
18 while being reflected back and forth between the bandpass filter
22 and the total reflection mirror 20. As a result, the angles of
reflection .theta.1, .theta.3, .theta.5, and .theta.7 are related
to one another as follows: .theta.1>.theta.3>.theta.5> and
.theta.7. Further, the exit angles .theta.1', .theta.3', .theta.5',
and .theta.7' described above are related to one another as
follows: .theta.1'>.theta.3'>.theta.5'>.theta.7'. It
should be noted that since the multiplexed optical signal includes
only four wavelengths .lamda.1, .lamda.2, .lamda.3, and .lamda.4,
no light is reflected from the bandpass filter 22 in the fourth
incidence event (that is, no light is reflected from the bandpass
filter 22 at the angle of reflection .theta.7).
[0035] The separated optical beams exiting the bandpass filter 22
then travel in the manner described below with reference to FIG. 6.
FIG. 6 is a diagram showing the paths traveled by these beams from
the bandpass filter 22 to the light receiving portions of the
photodetectors. As described above, the exit angles of the
separated optical beams from the bandpass filter 22 are related to
one another as follows:
.theta.1'>.theta.3'>.theta.5'>.theta.7'. Therefore, these
optical beams cross one another at intermediate locations between
the bandpass filter 22 and the photodetector array 26. The
condenser lens 24 is disposed so that these intermediate locations
are located within the lens 24. With this arrangement, the
condenser lens 24 focuses the separated optical beams exiting the
bandpass filter 22 (i.e., the four collimated optical beams having
wavelengths .lamda.1, .lamda.2, .lamda.3, and .lamda.4,
respectively) into focused beams. The focused beam having
wavelength .lamda.1 is then incident on the light receiving portion
of the photodetector 26a, and those having wavelengths .lamda.2,
.lamda.3, and .lamda.4 are incident on the light receiving portions
of the photodetectors 26b, 26c, and 26d, respectively.
[0036] As described above, the light receiving portions of the
photodetectors 26a, 26b, 26c, and 26d increase in area in the order
named (that is, the light receiving portion of the photodetector
26d is the largest in area and the light receiving portion of the
photodetector 26a is the smallest). The photodetector 26d receives
the optical beam (.lamda.4) which has been reflected three times by
the total reflection mirror 20; the photodetector 26c receives the
optical beam (.lamda.3) which has been reflected twice by the total
reflection mirror 20; the photodetector 26b receives the optical
beam (.lamda.2) which has been reflected once by the total
reflection mirror 20; and the photodetector 26a receives the
optical beam (.lamda.1) which has not been reflected by the total
reflection mirror 20. Thus, photodetectors having a light receiving
portion of larger area receive an optical beam which has been
reflected more times by the total reflection mirror 20 and hence
has traveled a longer optical path.
[0037] The construction of the multiple wavelength receiver module
of the first embodiment allows a single three-layer filter to
separate a multiplexed optical signal into individual optical
beams. Specifically, this is accomplished by use of the bandpass
filter 22 having angle-of-incidence dependent transmission
characteristics and the total reflection mirror 20 adapted to
reflect back light from the bandpass filter 22 so that the angle of
incidence of the light to the bandpass filter 22 changes with each
successive incidence. Further, the condenser lens 24 is disposed
where the separated optical beams cross one another, eliminating
the need for additional condenser lenses. Thus the construction of
the multiple wavelength receiver module 10 of the first embodiment
enables the manufacture of multiple wavelength receiver modules
without using many filters and condenser lenses, and hence at low
cost. Further, these multiple wavelength receiver modules are easy
to assemble, since they require alignment of the optical axis of
only one condenser lens.
[0038] The locations where the separated optical beams emerging
from the bandpass filter 22 cross one another can be adjusted as
desired by changing the exit angles of these beams from the filter
22. Changing the exit angles can be accomplished by adjusting the
thickness of the prism 18, the angle of the first surface 18a with
respect to the second surface 18b, and/or the angle of incidence of
the multiplexed optical signal incident on the prism 18. Therefore,
the condenser lens may be fixed at such a location that the module
can be easily assembled. With this arrangement, the locations where
the separated optical beams cross one another may be selected to be
within the condenser lens 24.
[0039] The photodetectors 26a, 26b, 26c, and 26d are monolithically
mounted in the photodetector array 26. Therefore, the optical axes
of these photodetectors can be aligned at once.
[0040] Incidentally, there may be a variation in the angle of the
multiplexed optical signal entering the prism 18 from the optical
fiber 14, depending on the usage of the multiple wavelength
receiver module. This results in displacement of the points of
incidence of the separated optical beams on the photodetectors. The
amounts of displacement, .DELTA.26a, .DELTA.26b, .DELTA.26c, and
.DELTA.26d, of the points of incidence of the optical beams on the
photodetectors 26a, 26b, 26c, and 26d, respectively, are related to
one another as follows:
.DELTA.26a<.DELTA.26b<.DELTA.26c<.DELTA.26d. That is,
optical beams which have traveled a longer path are displaced in
their point of incidence by a greater amount. If this amount of
displacement is too great, then the photodetector cannot receive
the optical beam, or there is a reduction in the optical coupling
efficiency.
[0041] On the other hand, the construction of the multiple
wavelength receiver module of the first embodiment ensures that the
photodetectors receive the optical beams emerging from the
condenser lens 24 even if there is a variation in the angle of the
multiplexed optical signal entering the prism 18. Specifically, in
the first embodiment, the light receiving portions of the
photodetectors 26a, 26b, 26c, and 26d increase in area in the order
named (that is, the light receiving portion of the photodetector
26d is the largest in area and the light receiving portion of the
photodetector 26a is the smallest). That is, light receiving
portions of larger area receive an optical beam displaced in its
point of incidence by a greater amount. Therefore, the multiple
wavelength receiver module has a high tolerance for variation in
the angle of incidence of the multiplexed optical signal to the
prism 18, so that the photodetectors can reliably detect the
optical beams emerging from the condenser lens 24, and so that the
multiple wavelength receiver module is easy to assemble.
[0042] FIG. 7 is a variation of the photodetector array of the
present embodiment. This photodetector array 30 differs from the
photodetector array shown in FIG. 3 in that photodetectors 30a,
30b, 30c, and 30d are substituted for the photodetectors 26a, 26b,
26c, and 26d. The light receiving portions of the photodetectors
30b, 30c, and 30d are elliptical in shape, and their major axes
extend in the direction in which these light receiving portions are
aligned. This configuration of the photodetector array 30 is
intended to accommodate displacement of the points of incidence of
the optical beams on the light receiving portions of the
photodetectors in a direction parallel to the direction in which
the light receiving portions are aligned. Since the light receiving
portions of the photodetectors 30b, 30c, and 30d are elliptical in
shape and their major axes extend in the direction in which these
light receiving portions are aligned, the photodetectors can
reliably detect the optical beams emerging from the condenser lens
24 even if the points of incidence of the optical beams on the
light receiving portions are displaced in the direction in which
the light receiving portions are aligned. As a result, the multiple
wavelength receiver module has a high tolerance for variation in
the angle of incidence of the multiplexed optical signal to the
prism 18, and furthermore the elliptical light receiving portions
of the photodetectors 30b, 30c, and 30d can be made smaller in area
than the circular light receiving portions of the photodetectors
26b, 26c, and 26d, respectively, of the present embodiment.
Therefore in this way the areas of the light receiving portions of
the photodetectors may be reduced to reduce the capacitances of the
photodetectors and thereby ensure that the multiple wavelength
receiver module has high speed response.
[0043] Although in the first embodiment the multiplexed optical
signal emitted from the optical fiber 14 includes light of 4
wavelengths, it is to be understood that the multiplexed optical
signal may include more or less than 4 wavelengths and the multiple
wavelength receiver module may include as many photodetectors as
there are wavelengths in the optical signal (or may include any
suitable number of photodetectors). Such constructions also have
the advantages described above in connection with the
invention.
[0044] FIG. 8 is a diagram showing a variation of the total
reflection mirror of the first embodiment. The total reflection
mirror 32 shown in FIG. 8 is a single-piece mirror with no openings
therein. In this case, the multiplexed optical signal is introduced
into the prism 18 through its wide upper end. This eliminates the
need to form an opening in the total reflection mirror 32,
resulting in reduced cost of the multiple wavelength receiver
module.
Second Embodiment
[0045] A multiple wavelength receiver module in accordance with a
second embodiment of the present invention has many common features
with the multiple wavelength receiver module of the first
embodiment. Therefore, the following description of the multiple
wavelength receiver module of the second embodiment will be
directed only to the differences from the multiple wavelength
receiver module of the first embodiment. FIG. 9 is a diagram
showing the light receiving surface of the photodetector array 34
of the multiple wavelength receiver module of the second
embodiment. The photodetector array 34 includes a substrate 34s and
photodetectors 34a, 34b, 34c, and 34d monolithically mounted on the
substrate 34s. The light receiving portion of the photodetector 34c
is circular in shape. The light receiving portions of the
photodetectors 34a, 34b, and 34d, on the other hand, are elliptical
in shape and their major axes extend in the direction in which
these light receiving portions are aligned. The light receiving
portion of the photodetector 34c is smaller in area than the light
receiving portions of the photodetectors 34a, 34b, and 34d. The
adjustment of the optical axes of all photodetectors is
accomplished by adjusting the optical axis of the light receiving
portion of the photodetector 34c.
[0046] As described above, a variation in the angle of the
multiplexed optical signal entering the prism 18 may result in
displacement of the points of incidence of the optical beams on the
light receiving portions of the photodetectors. In such cases,
light receiving portions closer to the adjusted optical axis
receive an optical beam displaced in its point of incidence by a
smaller amount. The optical axis of the entire photodetector array
34 is adjusted by adjusting the optical axis of the light receiving
portion of the photodetector 34c (as described above), which
portion is an intermediate one of the light receiving portions
linearly arranged on the substrate 34s. This means that the light
receiving portions of the photodetectors of the photodetector array
34 are spaced a relatively close distance from the adjusted optical
axis of the photodetector array 34 (which axis coincides with the
optical axis of the light receiving portion of the photodetector
34c), resulting in reduced displacement of the point of incidence
of the optical beam to each light receiving portion.
[0047] Further, the light receiving portion of the photodetector
34c is smaller in area than the light receiving portions of the
other photodetectors. This makes it difficult to align the optical
axis of the condenser lens 24 with the light receiving portion of
the photodetector 34c. However, the alignment of the optical axis
of the condenser lens 24 with the light receiving portions of all
photodetectors can be accomplished by aligning the optical axis
with the light receiving portion of the photodetector 34c alone.
Thus, the light receiving portion of one of the photodetectors may
be made smaller in area than the light receiving portions of the
other photodetectors, and the optical axis of the condenser lens 24
may be aligned with this light receiving portion to facilitate the
assembly operation (axis alignment operation) of the multiple
wavelength receiver module.
[0048] FIG. 10 is a diagram showing a variation of the
photodetector array 34. This photodetector array 36 is
characterized by the configurations of the light receiving portions
of its photodetectors 36a, 36b, 36c, and 36d. Specifically, the
light receiving portions of the uppermost photodetector 36d and the
lowermost photodetector 36a are smaller in area than the light
receiving portions of the other photodetectors. With this
arrangement, the alignment of the optical axis of the condenser
lens 24 with the light receiving portions of all photodetectors can
be accomplished only by aligning the optical axis with the light
receiving portions of the photodetectors 36a and 36d, since the
light receiving portions of the photodetectors 36b and 36c are
larger in area than the light receiving portions of the
photodetectors 36a and 36d. This facilitates the assembly operation
(axis alignment operation) of the multiple wavelength receiver
module.
Third Embodiment
[0049] FIG. 11 is a diagram showing a multiple wavelength receiver
module 50 in accordance with a third embodiment of the present
invention. This multiple wavelength receiver module 50 includes a
prism 52. A total reflection mirror 54 is bonded to a first surface
52a of the prism 52. The total reflection mirror 54 has an opening
54a. A bandpass filter 56 is bonded to a second surface 52b of the
prism 52 opposite the first surface 52a. The first surface 52a
extends at an angle with respect to the second surface 52b; that
is, the upper end of the prism 52 has a greater width than the
lower end of the prism 52. The prism 52, the total reflection
mirror 54, and the bandpass filter 56 are herein referred to
collectively as the three-layer filter.
[0050] A condenser lens array 58 is formed on the exit side of the
three-layer filter. The condenser lens array 58 includes
monolithically formed condenser lenses 58a, 58b, 58c, and 58d.
[0051] A photodetector array 60 is formed on the exit side of the
condenser lens array 58. The photodetector array 60 includes
monolithically formed photodetectors 60a, 60b, 60c, and 60d.
[0052] The propagation of light within the multiple wavelength
receiver module 50 thus constructed will be described. First, a
multiplexed optical signal is introduced into the prism 52 through
the opening 54a. That is, the multiplexed optical signal enters
into the narrow lower portion of the prism 52. The multiplexed
optical signal introduced into the prism 52 is separated, or
demultiplexed, into four individual optical beams in the same
manner as described above in connection with the first embodiment.
Therefore, this separation process will not be described herein.
The four separated optical beams are then incident on the condenser
lenses 58a, 58b, 58c, and 58d, respectively. The optical beams
exiting the condenser lenses 58a, 58b, 58c, and 58d are then
incident on the photodetectors 60a, 60b, 60c, and 60d,
respectively.
[0053] An application or specifications of a multiple wavelength
receiver module may require that the photodetectors be spaced a
certain distance from one another. In such cases, there is no way
to cause the optical beams emerging from the three-layer filter to
cross one another before they are incident on their respective
photodetectors, making it necessary to provide a condenser lens for
each of the separated optical beams. Mounting a plurality of
discrete condenser lenses in a multiple wavelength receiver module
complicates the assembly of the module. The multiple wavelength
receiver module of the third embodiment avoids this problem by
having a condenser lens array including a plurality of condenser
lenses, instead of having a plurality of discrete condenser lenses.
This array of condenser lenses can be mounted at once, resulting in
simplified assembly of the module.
[0054] Thus the present invention enables the manufacture of
low-cost multiple wavelength receiver modules which are easy to
assemble.
[0055] Obviously many modifications and variations of the present
invention are possible in the light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
[0056] The entire disclosure of a Japanese Patent Application No.
2010-160015, filed on Jun. 14, 2010 including specification,
claims, drawings and summary, on which the Convention priority of
the present application is based, are incorporated herein by
reference in its entirety.
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